This invention relates to the field of corrosion-resistant alloys and more particularly to low strategic metal content workable alloys resistant to both oxidizing and reducing sulfuric acid solutions over a wide range of acid concentrations.
For purposes of analyzing and predicting their corrosive effect on various metals, acids and other corrosive agents are commonly classified as either "oxidizing" or "reducing". A reducing medium is one in which the strongest oxidizing agent is the hydrogen ion or hydronium ion while an oxidizing medium includes components which are more highly oxidizing than either the hydrogen ion or hydronium ion. Sulfuric acid is normally a reducing acid but high strength sulfuric acid is often oxidizing, especially at elevated temperatures. Moreover, various industrial sulfuric acid streams contain various oxidizing acids and salts as contaminants. It is, therefore, desirable that an alloy designed for general utility in industrial sulfuric acid streams be resistant to both reducing and oxidizing environments.
Corrosion resistance of any given metal or alloy in a reducing medium is often sharply different from its resistance in an oxidizing medium with some metals and alloys being more resistant to reducing media and others to oxidizing media. These differences in behavior are thought to be attributable to differences between the corrosion mechanism in a reducing medium and the corrosion mechanism in an oxidizing medium. Thus, corrosive attack by a reducing acid is generally considered to involve attack on the metal by hydrogen ions resulting in the oxidation of metal to soluble ions and release of hydrogen gas. Metals of relatively high nobility, therefore, as indicated by their positions in the galvanic series, are generally resistant to corrosion by reducing acids. Attack by oxidizing media on the other hand does not involve release of hydrogen but commonly results in the formation of metal oxides or other metallic compounds at the metal surface. Unlike the situation with reducing acids, a favorable position relative to hydrogen in the electromotive series provides no insurance that a metal will not be rapidly attacked by an oxidizing medium. However, certain elements such as chromium, aluminum and silicon form tough insoluble oxide films on initial contact with an oxidizing medium and such films serve as barriers against further reaction between the medium and the metal, thus preventing further corrosion from taking place.
Sulfuric acid solutions are not only very corrosive generally but the nature of their corrosion properties varies markedly with both acid concentration and temperature. This variability relates at least in part to sulfuric acid's ambivalent assumption of both reducing and oxidizing properties as its concentration, temperature, and the nature and proportions of various contaminants are altered. As a consequence of this variability in its corrosive properties, few materials are available which are reasonably resistant to sulfuric acid solutions over a wide range of concentrations and temperatures. A relatively large number of available materials exhibit reasonable resistance to either dilute sulfuric acid solutions having an acid strength of less than about 20% by weight or to concentrated solutions having an acid strength greater than 80% by weight. A lesser number of materials are effective for the intermediate and generally more corrosive concentration range of 20-80%, and even fewer metals are commercially useful in contact with sulfuric acid solutions ranging from strengths below 20 to greater than 80%, particularly when exposed to elevated temperatures.
Of the known alloys which are demonstrably effective over wide ranges of sulfuric acid concentrations, many contain relatively high portions of nickel and chromium and are thus rather expensive. In my copending and coassigned U.S. patent application Ser. No. 463,886, filed Apr. 25, 1974, filed as a continuation-in-part of U.S. patent application Ser. No. 346,693, filed Mar. 30, 1973, which was in turn filed as a continuation-in-part of U.S. patent application Ser. No. 137,641, filed Apr. 26, 1971, sulfuric acid corrosion-resistant alloys are described in which the nickel content ranges between 22.1 and 52.1% by weight and the chromium content is quite low, ranging between 4 and 14.18% by weight. These are very desirable alloys, but have a fairly appreciable molybdenum content in the range of 4.77-17.9%. Another highly desirable alloy I have discovered is that described in my copending and coassigned U.S. patent application Ser. No. 399,687, filed Sept. 24, 1973. This alloy also has an appreciable molybdenum content, however, in the range of 6.7-14.5%. Molybdenum is a fairly scarce and expensive metal whose presence in significant quantities materially contributes to the overall cost of the alloy.
Johnson U.S. Pat. No. 3,758,296 discloses a relatively low molybdenum content alloy comprising 26-48% nickel, 30-34% chromium, 4-5025% molybdenum, 4-7.5% cobalt, 3-25% iron, 2.5-8% copper, 0.05-0.25% carbon, up to 4% silicon and up to 0.10% boron. Silicon in the range of 2-3.5% is said to be preferred. The alloys disclosed by Johnson, however, exhibit rather high hardness, not only because of the preferred 2-3.5% silicon content, but also because of the required presence of 4-7.5% cobalt. The alloys of the Johnson patent are designed to be susceptible to precipitation hardening, a two-step process in which the alloy is first subjected to solution heat treatment followed by rapid quenching, and then to a precipitation or aging treatment which causes separation of a second phase from the solid solution, attended by hardening of the alloy. Because of the relatively high hardness and high yield strength which they exhibit, the alloys of the Johnson patent are primarily adpated for use in castings and are not readily susceptible to working into wrought forms.
A continuing need has, therefore, existed for corrosion-resistant workable alloys having a relatively low strategic metal content. In particular, a need has existed for such alloys in which the nickel and chromium contant is relatively low, since nickel and chromium are both expensive metals supplied almost exclusively from sources outside the United States. At the same time, there has been a need for such alloys which are not only low in nickel and chromium but also have the lowest feasible proportions of other expensive components such as molybdenum, tantalum, tungsten, vanadium and niobium.